S4 CHEMICAL COMPOSITION OF SOILS 



Before plants can secure this nitrogen for their growth the organic 

 matter must decay. In a similar manner the mineral soil particles 

 must decay or partially dissolve before plants can obtain the 

 essential mineral elements. 



Lime, Its True Meaning. — ^Lime is the oxide of the element 

 calcium, or calcium oxide (CaO). Pure burnt lime or quick-hme 

 is nothing else than calcivim oxide. Lime does not exist in soils 

 in the form as indicated by its symbol; it is to be found in more 

 complex substances. It is to be remembered that the calcium con- 

 tent of any substance may be expressed as per cent calcium (Ca) 

 or as per cent lime (CaO). 



Potash, Its Common Meaning. — The word potash is commonly 

 used to mean the oxide of the element potassium, or potassium 

 oxide (K20).^ Its true meaning, however, is potassium carbonate, 

 a soluble alkaU salt obtained by leaching wood ashes. True 

 potash is not a substance that can be represented by the formula 

 (K2O) ; it is a more complex substance, as indicated by the for- 

 mula K2CO3. 



The potassium content of soils or of any fertilizing material 

 may be expressed as per cent potassium (K), and commonly as 

 per cent '^ potash" (K2O). 



" Phosphoric acid " is the name frequently given to the oxide 

 of the element phosphorus, (P205).^ It does not exist in nature; 

 nevertheless it can be made in a chemical laboratory. 



The phosphorus content of soils or of any fertilizing substance 

 may be expressed as the element phosphorus (P), and as it is 

 frequently expressed, as phosphoric acid (P2O6). 



Chemical Composition of Soils Influences Plant Growth. — 

 This brief discussion concerning the chemical composition of soils 

 is sufficient to give us the idea that a loam, silt loam or any other 

 class or type of soil is a complex chemical substance down ipto 

 which land plants send their roots, not only to anchor themselves 

 but to obtain water and elements necessary for growth. It is 

 reasonable, therefore, to believe that the chemical composition 

 of soils influences the growth and development of plants in a 

 greater or less degree (Fig. 15). 



Chemical Composition of Plants Similar to That of Soils.— For 

 centuries no one knew just what plants were made of or how they 



^ The formula K2O means two atoms of potassium united with one atom 

 of oxygen. 



4 The formula P2O5 signifies the union of two atoms of phosphorus with 

 five atoms of oxygen. 



